1. Field of the Invention
The present invention relates to an optical communication system which uses time division multiplexing in upstream communication. The present invention can be applied to an optical communication system for providing a subscriber with services such as FTTH (Fiber To The Home).
2. Description of Related Art
As conventional optical communication systems, there is a known system in which an upstream signal carrier (i.e., a carrier used to send a signal from a terminal unit to a network unit) is generated by the network unit side and the upstream signal carrier is supplied to the terminal unit. Such an optical communication system is disclosed in, for example, paragraphs 0002 to 0008 and FIG. 14 of Japanese Patent Application Laid-Open No. 2004-222255. Japanese Patent Application Laid-Open No. 2004-222255 discloses that, in a Wavelength Division Multiplexing (WDM) optical communication system, a downstream signal light and an upstream optical carrier are wavelength-multiplexed and the wavelength-multiplexed light is sent to a terminal unit (“ONUs 70-1 to 70-n” in the document) from a network unit (“OSU 50” in the document). The terminal unit demultiplexes the upstream signal optical carrier from the received wavelength-multiplexed light and uses the upstream signal optical carrier to generate an upstream signal light. According to the optical communication system of Japanese Patent Application Laid-Open No. 2004-222255, a light source used to generate an upstream signal carrier does not need to be provided to individual terminal units and a light source provided in the network unit can be shared between the terminal units. By thus sharing the light source, the costs of terminal units can be reduced; consequently, the construction costs of the entire system can be reduced.
In addition, as conventional optical communication systems, there is a known system that uses Time Division Multiplexing (TDM) in upstream communication. Such an optical communication system is disclosed in, for example, paragraph 0003 and FIG. 7 of Japanese Patent Application Laid-Open No. 11-298430. In the case of using Time Division Multiplexing in upstream communication, timing needs to be controlled so that signals sent from different terminal units (subscriber optical terminals 15 in the document) do not collide with each other. Hence, in the system of Japanese Patent Application Laid-Open No. 11-298430, a control signal for controlling upstream transmission timing is sent to the terminal units from a network unit (station side device 14 in the document). In the optical communication system, the distance from the network unit to the terminal units varies from terminal unit to terminal unit. Therefore, in order to prevent a collision between upstream signals, the transmission timing for each terminal unit needs to be controlled taking into account the difference in distance.
As described above, the system of Japanese Patent Application Laid-Open No. 2004-222255, since Wavelength Division Multiplexing is used in upstream communication, multiple types of wavelengths need to be used; as a result, the system construction costs are still expensive. Hence, when a large band is not required, it is desirable to use Time Division multiplexing in upstream communication.
However, as described above, in an optical communication system that uses Time Division Multiplexing in upstream communication, the transmission timing needs to be controlled, taking into account the difference in distance from the network unit to each terminal unit (see Japanese Patent Application Laid-Open No. 11-298430). Such timing control makes the processes performed by the network unit and the terminal units complicated, and accordingly, the system costs cannot be sufficiently reduced.